Feature request: multiple arrowheads along the edge of a directed arc

[hdavid16]

This might be something interesting to have at some point. The user could specify the relative spacing between arrowheads along the length of the edge. Another option would be to specify waypoints for the arrowheads.

This is obviously not a must have, but would be a feature that would give more flexibility to the user.

General idea: have an attribute (e.g., arrow_number) such that this number of arrowheads are added. We could leverage the interpolate function and apply it over several values. For example: if arrow_number = 3, then apply interpolate at 0.25, 0.50, and 0.75 (i.e., interpolate(p,t) for t in range(0,1,arrow_number+2)[2:end-1]).